The present invention relates to a power steering system for vehicles. In particular, the present invention relates to a speed proportional power steering system.
Speed proportional steering systems for vehicles are known. One of the known speed proportional steering systems includes first and second valve members rotatable relative to each other to port fluid to a power steering motor. An increased effort is required to move the first valve member relative to the second valve member as the speed of the vehicle increases.
U.S. Pat. No. 4,034,825 discloses one example of a steering gear in which a higher steering effort is required as vehicle speed increases. A torsion bar located in a power steering valve has balls circumferentially spaced therearound at a location intermediate portions of the torsion bar which are connected to valve members of the power steering valve. The balls are located in V-shaped notches in the torsion bar. Fluid pressure is applied to the balls to force the balls into engagement with surfaces of the V-shaped notches. When the fluid pressure acts on the balls, the location at which force is transmitted between the torsion bar and a valve member changes thereby changing the effective length of the torsion bar. Thus, the effective spring rate of the torsion bar changes to increase the steering effort required to turn the steerable wheels. The fluid pressure is controlled to increase with vehicle speed. This system has the disadvantage that it requires a hydraulic fluid source and mechanical interaction to effect speed proportional steering. These fluid and mechanical features are relatively complex and increase the overall cost of the steering gear.